When operating wireless networks, a problem can occur when multiple networks or multiple devices want to operate over the same channel using the same bandwidth. Some sort of scheme must be implemented to separate the networks in some way so that transmissions from one will not interfere with transmissions from the other.
One option for handling multiple networks or devices is to use a frequency division multiple access (FDMA) scheme. Such an implementation is often used in narrow band systems. In an FDMA scheme, different networks or devices are separated by each being assigned a different frequency band. Each network or device then gets to use its assigned portion of the spectrum and can be assured that other networks will not interfere with that assigned frequency portion. An example of this is FM radio.
However, this is not readily applicable to a UWB implementation since UWB by its nature uses wide frequency bands for its signals. In fact with UWB systems, it is generally advantageous to use as wide a spectrum as possible for transmissions. Because of the need to use very wide frequency bands, it is sometimes not feasible to break up the available spectrum into smaller, mutually exclusive frequency bands.
Another option for handling multiple networks is to use a code division multiple access (CDMA) scheme. In a CDMA scheme networks and devices transmit over the same frequency spectrum and at the same time, but signals from each are encoded using codes specially chosen to minimize their interference with each other.
However this kind of a scheme also has limitations. First, there are only so many codes that have the desired isolation properties needed to keep overlapping networks and devices separate, thus limiting the number of networks or devices that can operate at the same time in a given area. Second, no matter how good the code separation is, it isn't perfect. There is always some bleed over into transmissions from other networks. As a result of this, a close device of a different network can often drown out a distant device of the same network, despite the fact that the codes used by the close device are chosen to minimize interference with the other network. This can be referred to as the near-far problem.
Yet another option for handling multiple networks is a time division multiple access (TDMA) scheme. In a TDMA scheme, the available transmission time is broken up into multiple time slots, and each network or device is assigned one or more of the time slots. Thus, each device is given some portion of the available transmission time to use and is forced to remain silent during all other times.
However, this TDMA scheme forces each network or device to reduce its speed, since it isn't allowed to transmit during the entire available channel time. And as the number of overlapping networks or devices increases, transmission speed will be correspondingly reduced. For example, if there is 100 megabits per second (Mbps) capacity divided evenly over four separate networks, each network would be limited to a 25 Mbps transmission speed.
One way to transmit more data in a TDMA scheme is to increase the transmission power for a given network or device. In a digital system, for example, using a stronger signal means that each individual bit of data requires less time to send, enabling the device to operate at an increased data rate. A significantly increased transmit power can, therefore, compensate for time lost when other networks or devices are transmitting.
However, this solution of increasing transmit power has limited application to UWB systems. In the United States the Federal Communications Commission (FCC) has imposed a limit on the maximum allowable transmit power for UWB signals. And there is every reason to believe that similar agencies in other countries will impose similar restrictions. This in turn represents a limit on the maximum capacity for the combination of all available networks.
Therefore, if a TDMA scheme is used for UWB signals, it would be desirable to maximize the transmit power used for any given signal, while minimizing that signal's width in the time domain. And it would be desirable to achieve this result without violating the maximum signal power restrictions set up by the FCC or similar regulating agency. This would allow each network or device to maximize its data transmit rate while minimizing the portion of available transmit time that it used.